1. Field of the Invention
This invention relates to novel catalysts for the polymerization of olefins. The invention especially relates to a process for the production of polyolefins having a broad molecular weight distribution characterized in that a cocatalyst comprising isoprenylaluminum and a halogenated hydrocarbon is used together with a transition metal compound of Groups IVb to VIb of the Periodic Table (56th Edition of Handbook of Chemistry and Physics, CRC Press 1975). More particularly, the invention is concerned with the cocatalyst comprising isoprenylaluminum and a halogenated hydrocarbon which is employed together with a catalyst comprising a transition metal compound of a Group IVb to VIb metal in a polymerization process for the production of polyethylene having a broad molecular weight distribution.
2. Description of the Prior Art
It is known that the molecular weight of polyethylene may be controlled by the addition of a controlled amount of a haloalkane to a catalyst system comprising a catalyst of a compound of a metal of Groups IVb, Vb, and VIb and an organometallic compound of an alkali metal, alkaline earth metal, zinc, earth metal and especially aluminum. For example, Vandenberg in U.S. Pat. No. 3,354,139, issued Nov. 21, 1967, discloses the use of haloalkanes in combination with a transition metal compound and an organoaluminum compound. Favis discloses in U.S. Pat. No. 3,472,910 that tertiary alkyl halides and alkyl halocyclopentanes can be usefully employed in combination with a Ziegler-type catalyst in order to obtain a low molecular weight liquid linear alpha-olefin product.
Halohydrocarbons have been employed for improving tacticity of polymer product and activity of catalyst. In, for example, U.S. Pat. No. 3,432,513 of Miller et al, issued Mar. 11, 1969, it is disclosed that certain halogenated hydrocarbons can be usefully employed in combination with a Ziegler-type catalyst comprising an organoaluminum compound and a titanium metal halide for the production of highly isotactic polypropylene. Each of U.S. Pat. Nos. 4,182,691, 4,183,826, and 4,242,230 of Ueno et al discloses an improved titanium trichloride containing catalyst system obtained by treating a titanium trichloride produced by reducing titanium tetrachloride with an organo-metal compound and contacting the resulting reduced solid product with a chlorinated saturated hydrocarbon. The obtained titanium trichloride complex is usefully employed with an organoaluminum cocatalyst for the polymerization of alpha-olefins. Lowery et al in U.S. Pat. No. 4,250,288, issued Feb. 10, 1981, discloses a catalyst system exhibiting high catalytic activity in the polymerization of alpha-olefins, said catalyst system comprising a transition metal compound such as, tetra(isopropoxy)titanium and an organomagnesium component such as a hydrocarbon soluble complex of dialkyl magnesium and an alkyl aluminum and an active hydrocarbyl halide.
Each of U.S. Pat. Nos. 4,004,071, 3,989,878, 4,159,965 and 4,027,089, assigned to Asahi Kasei Kogyo Kabushiki, discloses ethylene polymerization catalysts comprising the reaction product obtained by contacting a titanium compound with a hydrocarbon-soluble organomagnesium-aluminum complex and an organoaluminum compound cocatalyst. The patentees teach that in order to control the molecular weight of the obtained polymer it is possible to add, among others, a halohydrocarbon or hydrogen to the reaction system.
In U.S. Pat. No. 4,355,143 of Lassalle et al, molecular weight distribution is controlled through the use of halogenated ethylenic hydrocarbons as a catalyst modifier.
It is known that olefin polymerization catalysts comprising titanium as the primary active polymerization cite yield resins which have a narrow molecular weight distribution (M.sub.w /M.sub.n less than about 10). For many applications of polyethylene it is important to improve the toughness properties of the processed polymer. A method of achieving this increase in toughness is accomplished by preparing polymers of high molecular weight (low melt index). There is however at these high molecular weight values, a corresponding decrease in rheological properties. The decrease in rheological properties can be compensated for by broadening the molecular weight distribution of the polymer in order to improve processing characteristics.
For example, polyethylene having a broad molecular weight distribution is advantageously processed during the extrusion process, and in blow molding, polyethylene having a broad molecular weight distribution obtains an extruded product of increased strength.
In view of the foregoing problems encountered in the use of Ziegler catalysts, it would be highly desirable to provide a polymerization catalyst which produces polyolefins and particularly polyethylene with high molecular weight and broad molecular weight distribution.